Proteolysis of von Willebrand factor (VWF) normally occurs through the action of a plasma enzyme that has recently been characterized (ADAMTS13); it accounts for the small quantities of cleavage products normally present in the circulation, and its inhibition can lead to the disease called thrombotic thrombocytopenic purpura (TTP). We developed a rapid assay to evaluate the cleavage of VWF and have characterized patients with a TTP-like syndrome to detect those with low VWF cleaving protease activity. The assay does not require specialized reagents and can be completed within 6-8 hours on patient plasma. We participated in an international study testing coded samples with varying levels of ADAMTS13 to compare our method with other more quantitative methods and found that we could differentiate normal from abnormal values in 95% of samples. We also studied a specific patient in detail who had SLE and an episode of TTP in whom the diagnosis was problematic due to the underlying disease. The level of protease activity and its inhibitor aided in the diagnosis and clinical management, including the duration of plasma exchange, in this patient. The assay has also been used to follow treatment of TTP patients using rituximab.